Conventionally, respiratory assistance consists in ventilating a patient using a pressurized gas, for example air or oxygen-enriched air, that is to say, during the inhalation phase which is usually initiated by the patient, in applying a constant positive pressure in the "patient" circuit of a breathing appliance.
The "patient" circuit usually consists of ducting elements which make it possible to connect the patient's airways to the pressurized gas source; the patient circuit therefore comprises elements such as the breathing duct or ducts, a breathing mask or goggles, and a tracheotomy tube.
The pressure in the patient circuit varies depending on the respiratory phase: inhalation phase or exhalation phase. Specifically, during the inhalation phase, the pressurized gas output by a gas flow source, or turbine, is distributed to the patient's airways at a given inhalation pressure, whereas during the exhalation phase, the exhalation initiated by the patient is passive and takes place at atmospheric pressure or at a given positive exhalation pressure, or PEP; the exhaled gases are discharged either through one (or more) exhalation valves arranged on the patient circuit, or through holes or ports arranged in the breathing mask.
Respiratory assistance devices operating on this principle are commonly referred to as respiratory assistance devices having two pressure levels.
It has been observed that discharging the exhalation gases, which are rich in CO.sub.2, via holes or ports made in the breathing mask had the major drawback of not allowing full discharge of the exhaled gases, which tend to build up in certain parts of the patient circuit, in particular in the breathing mask. During the following inhalation phase, a build-up of this type leads to re-inhalation of the gases which are rich in CO.sub.2, which is a problem for the patient.
Respiratory assistance devices having two pressure levels which are equipped with one or more exhalation valves are therefore preferred.
Mention may be made of EP-B-0,317,417, which describes a respiration assistance device in which the patient circuit comprises a pneumatically controlled exhalation valve. During the inhalation phase, the control inlet of this valve is subjected to the pressure of a pressurized flow source which closes the exhalation valve and allows the patient circuit to be connected in a leak-tight fashion to the pressurized gas flow source. When a significant reduction in the inhaled flowrate is detected, control electronics fully suspends the operation of the pressurized flow source, the structure of which is such that its outlet orifice is then returned to atmospheric pressure, which pressure is applied to the exhalation valve and thus allows it to open.
EP-A-0,425,092 relates to a respiration aid device which, in place of the exhalation valve, includes a calibrated permanent-leakage orifice, while the pressure of a flow source is set to two different levels depending on whether the phase is an inhalation phase or an exhalation phase.
In other words, this type of known device of the prior art has a mode of operation which is based on at least partially, and generally fully, interrupting the gas flow source during the exhalation phase. However, any stopping of the gas flow source has a major drawback, namely that when changing from an exhalation phase to the next inhalation phase, the pressure rise time of the patient circuit depends closely, on the one hand, on the power of the gas flow source and, on the other hand, on its mechanical inertia.
The result of this is that, in order to perform well and be efficient, and to overcome these problems, these devices necessarily need to be provided with a powerful gas flow source having little mechanical inertia. However, the gas flow sources currently available on the market which make it possible to meet such specifications of power and inertia, usually have drawbacks which are incompatible with use at home, and more generally in the medical field: excessive bulk, noise pollution, etc.
Furthermore, known respiratory assistance devices also sometimes have another drawback, namely that they do not allow the exhaled gas to be discharged properly.
Indeed, in order to prevent the exhalation gas, which is rich in CO.sub.2, from passing along the patient circuit during the exhalation phase, and then being re-inhaled during the following inhalation phase, it is recommended for a flow of gas to be maintained in the patient circuit, between the flow source and the patient; this flow makes it possible to remove the exhaled gases.
It is easy to see that, if the communication between the gas flow source and the patient is fully interrupted during the exhalation phase, there can be no circulation gas in the patient circuit, and there is a risk of the exhalation gases building up.
A partial solution to these problems is provided by WO-A-94/06499, which describes a respiration aid device comprising a patient circuit having an inhalation branch connected to a pressurized inhalation flow source, and an exhalation branch equipped with an exhalation valve which is controlled in such a way that it is closed during inhalation.
According to a first embodiment of this device, drive means control, on the one hand, distribution means and, on the other hand, the interruption of the communication between the inhalation flow source and the inhalation branch of the patient circuit, when a sensor detects that the patient is preparing for an exhalation phase; the communication between the inhalation flow source and the inhalation branch is reestablished when changing to an inhalation phase, by controlling the distribution means. In other words, throughout the exhalation phase, the inhalation flow source is kept in operation such as to provide the inhalation flow.
It is easy to see that this embodiment is not satisfactory because, during the exhalation phase, the flow source continues to distribute pressurized gas, and this will build up in the patient circuit between the flow source and the site where the communication is interrupted. This build-up of gas will result in an upstream overpressure of the patient circuit, and on entering the following inhalation phase and after re-establishing the said communication, this overpressure will propagate to the patient's lungs, which will then suffer a harmful "pulmonary respiratory shock" due to this overpressure. Furthermore, since the gas distributed by the flow source during the exhalation phase is not removed, the said flow source is subject to damage, in particular through heating. Finally, this embodiment does not solve the problem of the exhaled gases building up, since the communication between the flow source and the breathing mask is interrupted during the exhalation phase.
In order to try to overcome these problems, WO-A-94/06499 proposes a second embodiment, implementing a leakage-compensation duct which, during the exhalation phase, makes it possible to channel a fraction of the gas delivered by the respiratory source to a site in the patient circuit lying downstream of the site where the communication is interrupted. However, although this solution allows a partial solution to the problems of damage to the flow source and removing the exhaled gases, it nevertheless has several other drawbacks.
Specifically, the bypass duct has an unalterable constant diameter and is not therefore suited to all values of flow rate, for example according to whether the patient to be ventilated is an adult or child.
Thus, when the gas flow delivered by the flow source becomes too great for the bypass duct to fulfil its role properly, because its diameter is too small in comparison with the flow rate, the above problems of overpressure, risks of "pulmonary respiratory shock" and damage to the flow source again arise. Conversely, when the delivered gas flow rate becomes less than a certain threshold value, the bypass duct will allow an excessive quantity of gas to pass to the patient during the exhalation phase, and this will counteract proper exhalation and satisfactory removal of the gas, which is enriched in CO.sub.2, exhaled by the patient.